(1) Field of the Invention
The present invention relates to a process for the production of multi-layer pipes for draw-forming. More particularly, the present invention relates to a process in which a mutli-layer pipe to be used for the production of a draw-blow-formed bottle comprising a polyester layer and an oxygen-barrier resin layer is prepared in the transparent state while preventing crystallization.
(2) Description of the Prior Art
A process for preparing bottles by pre-heating a preform of a polyester composed mainly of ethylene terephthalate units to a drawing temperature of, for example, 85.degree. to 110.degree. C. and biaxially drawing the heated preform in the axial direction and the circumferential direction has been broadly adopted in the art.
In preparing a preform to be used for draw-forming, it is important that a resin molded in a molten state should be rapidly cooled to a super-cooled state to inhibit crystallization, for if crystallization is caused in the preform, the draw-formability is reduced and a final bottom becames opaque.
Draw-blow-formed polyester bottles prepared according to the above-mentioned process have been broadly used as packaging vessels for carbonated drinks, beer, whisky, soy sauce, other sauces and detergents because they have such preferred characteristics as high toughness, light weight, high transparency, high pressure resistance and act as a good gas barrier. However, even polyester bottles which are regarded as being excellent in the gas barrier property among plastic vessels, are considerably inferior to metal cans or glass bottles as gas barrier vessels for storing and maintaining carbonated drinks or fruit juice drinks. Namely, although a metal can wall or a glass bottle wall has a gas permeability of zero, a polyester allows certain permeation of a gas such as oxygen gas or carbon dioxide gas even if the polyester is excellent in the gas barrier property. Namely, although drawn polyester bottles recently developed have excellent characteristics not possessed by other materials, such as high transparency, light weight, high toughness and high pressure resistance, they cannot be regarded as being completely satisfactrory in the gas barrier property. Accordingly, the shelf life of a drawn polyester bottle filled with a carbonated drink, beer or fruit juice is somewhat limited or the use of a drawn polyester bottle is limited to a large-size vessel which is placed under a mild storage condition. Therefore, drawn polyester bottles are not satisfactory as vessels for circulation of bottled goods.
Under this background, improvement of the gas barrier property of a polyester vessel is inevitably desired, and since a vessel composed of a single layer of a polyester is not satisfactory, it is desired to impart a higher gas barrier property by a better method.
As the method for improving the gas barrier property in a vessel composed of a single layer of a polyester, there are considered, for example, a method in which the surface of a polyester vessel is covered with an aluminum foil or glass foil having no gas permeability, a method in which the inner and outer surfaces of a polyester vessel are coated with a gas-barrier resin lacquer, and a method in which in preparing a preform or pipe for draw-forming, a multi-layer structure is formed by co-injection (inclusive of multi-staged injection) or co-extrusion using a gas barrier thermoplastic resin.
Of multi-layer structure-forming methods, the coating or covering method is defective in that peculiar troublesome operations are necessary in addition to the bottle-forming operation, and the method for forming a multi-layer preform by multi-staged injection is troublesome and defective in that molds should be exchanged according to the kinds of resins and injection should be carried out in multiple stages. In contrast, the co-extrusion method for preparing a pipe for draw-forming is advantageous in that it is sufficient if a plurality of molten resin streams are joined together in a die and hence, the operation is very easy.
As pointed out hereinbefore, a multi-layer drawn polyester bottle is advantageous in that the formed bottle is transparent. Accordingly, it is important that the starting multi-layer polyester pipe should also be transparent.
Formation of transparent multi-layer polyester pipes, however, involves several difficulties. In case of extrusion molding of a pipe composed of a single layer of a polyester, a considerably transparent pipe can be easily obtained, though the thickness is limited to some extent. However, in case of a multi-layer polyester pipe intended in the present invention, formation of a transparent pipe is not so easy. It has been found that the main cause is that a gas barrier resin such as an ethylene/vinyl alcohol copolymer, which is interposed as an intermediate layer, acts as a barrier to conduction of heat when a molten multi-layer polyester pipe is cooled.
More specifically, cooling of a pipe is performed by introducing an extrudate into water as in case of cooling of a film or sheet. In case of a pipe, even though the outer side of the pipe falls in contact with cooling water, since introduction of cooling water into the inner side of the pipe is difficult, the inner side of the pipe cannot but be effected by cooling of a low cooling efficiency such as heat conduction cooling from the outer side or cooling in the gas phase.
Especially in the case where an ethylene/vinyl alcohol copolymer is combined with a polyester in forming a multi-layer structure so as to improve the gas barrier property in a polyester bottle, there is adopted a method in which the ethylene/vinyl alcohol copolymer in which the gas barrier property is reduced by moisture is interposed as an intermediate layer and sandwiched with inner and outer layers of polyethylene terephthalate. Among various resins, the ethylene/vinyl alcohol copolymer has a lowest heat conductivity, and this copolymer acts as a barrier to conduction of heat. Therefore, it often happens that the inner polyester layer located on the inner side of the pipe is hardly cooled, with the result that crystallization is caused in the inner polyester layer and only a pipe having a hazy inner layer is obtained.